1. Field of the Invention
The present invention relates to a solar heat collector and more particularly to a technology which enhances the collector efficiency of a solar heat collector by reducing thermal radiation therefrom.
2. Description of the Related Art
Solar heat collectors are such that a supplied heat medium such as water or an anti-freeze is heated up by sunlight and are used as, for example, a hot water supply source or a heat source. As such solar heat collectors, a solar heat collector of a flat plate-type is known in which an absorber plate which converts received sunlight into heat is placed in a box-shaped case while being supported by a thermal insulation material, and an upper opening in the case is covered by a light transmitting member such as a glass plate or a transparent resin plate, whereby sunlight which passes through the light transmitting member is converted into heat by the absorber plate to thereby heat a heat medium which flows in a flow path which is provided in contact with the absorber plate. In order to enhance the collector efficiency of the flat plate-type solar heat collector, the light transmittance of the light transmitting member and the photo-thermal conversion rate (thermal collection rate) of the absorber plate itself need to be enhanced while suppressing heat radiation from the light transmitting member as well as heat radiation from a side plate and a bottom plate of the case.
Although it is considered, for example, to increase the thickness of an insulating material which is provided on the side plate and the bottom plate of the case in order to suppress the heat radiation from the case and the like, a problem is caused that the overall size of the solar heat collector is enlarged. In addition, it is considered to create a vacuum within the case so as to reduce the radiation of heat collected to the absorber plate to the atmosphere from outer surfaces of the case and the light transmitting member, the case and the light transmitting member have to be constructed so as to withstand the atmospheric pressure. Because of this, the materials of the case and the light transmitting member need to be reinforced, and the case and the light transmitting member need, for example, to be reinforced by a spacer, leading to a problem that the weight of the heat collector is increased or the available absorber area is reduced. Further, the vacuum inside the case needs to be maintained over a long period of time, leading to a problem with a seal at the joint portion between the light transmitting member and the case.
Under the background described above, in order to suppress the radiation of heat from a case of such a flat plate-type solar heat collector, according to JP-A-62-294849, it is described that a thermal insulation material such as glass wool is fully laid at a bottom plate portion of a case and that a side plate of the case is formed of a thermal insulation material. In addition, in JP-A-2004-278837 it is implied that a bottom plate and a side plate of a case are formed of a vacuum thermal insulation material so as to reduce the radiation of heat from the bottom plate and the side plate of the case.
Further, JP-A-2010-127463 proposes a vacuum thermal insulating panel which belongs not to the technical field of solar heat collectors but to the technical field of cold thermal insulation for refrigerators or freezers. In this vacuum thermal insulating panel, two metal panels are worked into corrugated panels of a continuous triangular or sine wave. Then, the two metal panels so formed are disposed so that respective ridge lines of the corrugated panels intersect each other at right angles, while root lines of one of the two corrugated panels are in point contact with root lines of the other corrugated panel. Circumferential edges of the two metal panels are bonded together through welding or the like, and a vacuum is created in spaces defined by the two metal panels. According to this technique, the vacuum thermal insulating panel can hold rigidity against internal negative pressure or vacuum and rigidity against the overall deflection or deformation of the panel, and the necessity of a core material, a reinforcement material or a frame material for rigidity reinforcement can be obviated.
Additionally, according to a solar heat collector described in JP-A-2007-24298, a light transmitting member of a vacuum thermal insulating type is proposed. In this light transmitting member, in order to reduce the radiation of heat from the light transmitting member, pyramidal grooves or conical grooves are formed in an inner surface side of one glass plate of two glass plates which constitute a double grazing construction of the light emitting member so as to allow the glass plate to protrude on the inner surface side to thereby form sealed spaces where a vacuum is created. According to this proposal, the vacuum thermal insulating type light transmitting member can be realized which is prevented from collapsing that would otherwise be the case due to internal vacuums when the atmospheric pressure is applied thereto.
In addition, according to JP-UM-A-3-89351, a light transmitting member of a glass tube type is known in which a heat medium tube which doubles as an absorber plate is inserted concentrically in an inner glass tube of two glass tubes making up a double tube construction of the light transmitting member, and a vacuum is produced in a space defined between the two glass tubes so as to suppress the radiation of heat. Among such glass tube type light transmitting members, there is known a light transmitting member in which a heat medium tube having a flat plate-shaped collector fin is inserted concentrically in a glass tube of a single tube construction, and a vacuum is created in a space defined between the glass tubes so as to suppress the radiation of heat.
On the other hand, in JP-A-2004-278837 described above, a technique is proposed in which an inactive gas having a low thermal conductivity is filled in a space defined between two glass panels making up a double grazing construction of a light transmitting member in place of creating a vacuum in the space so as to reduce the radiation of heat from the light transmitting member.